1. Field of the Invention
It is desirable in many olefin polymerization processes, particularly a slurry phase or gas phase process, to use a supported catalyst system. A particularly useful catalyst system typically includes a metallocene and an alumoxane supported on a carrier, such as silica. For example, U.S. Pat. No. 4,937,217 generally describes a mixture of trimethylaluminum and triethylaluminum added to an undehydrated silica to which a metallocene catalyst component is then added. EP 308177 B1 generally describes adding a wet monomer to a reactor containing a metallocene, trialkylaluminum and undehydrated silica. U.S. Pat. Nos. 4,912,075, 4,935,397 and 4,937,301 generally relate to adding trimethylaluminum to an undehydrated silica and then adding a metallocene to form a dry supported catalyst system. U.S. Pat. No. 4,914,253 describes adding trimethylaluminum to undehydrated silica, adding a metallocene and then drying the resulting supported catalyst system with an amount of hydrogen to produce a polyethylene wax. U.S. Pat. Nos. 5,008,228, 5,086,025 and 5,147,949 generally describe forming a dry supported catalyst system by the addition of trimethylaluminum to a water impregnated silica to form alumoxane in situ and then adding the metallocene. U.S. Pat. Nos. 4,808,561, 4,897,455 and 4,701,432 describe techniques to form a supported catalyst where the inert carrier, typically silica, is calcined and contacted with a metallocene(s) and an activator/cocatalyst component. U.S. Pat. No. 5,238,892 describes forming a dry supported catalyst system by mixing a metallocene with an alkyl aluminum and then adding undehydrated silica. U.S. Pat. No. 5,240,894 generally pertains to forming a supported metallocene/alumoxane catalyst system by forming a metallocene/alumoxane reaction solution, adding a porous carrier, and evaporating the resulting slurry to remove residual solvent from the carrier.
While all these supported catalysts are useful, it would be desirable to have an improved metallocene catalyst system which in producing polymers does not foul the reactor. Particularly in a slurry or gas phase polymerization processes using these catalyst systems, there is a tendency for reactor operation problems during polymerization. During a typical polymerization process, fines within the reactor often accumulate and cling or stick to the walls of a reactor. This is the first stage of a phenomenon that is often referred to as “sheeting.” After a relatively short period of time during polymerization, sheets formed from the aggregation of the fines begin to appear in the reactor, and these sheets plug product discharge systems forcing shutdown of the reactor. The sheets so formed may vary widely in size, but are similar in many characteristics. They consist of fused polymer which is oriented in the long direction of the sheets and their surfaces have a granular resin which has fused to the core. They are essentially strands of fused polymer.
The accumulation of polymer particles on the reactor surfaces and walls of the recycling lines, distributor plate if employed, and cooling system results in many problems. Of particular importance is the problem of poor heat transfer during the polymerization process. Polymer particles that adhere to the walls of the reactor can continue to polymerize and often fuse together, forming growing aggregate masses, which can be detrimental to a continuous and batch polymerization processes when they become sufficiently large. These aggregate masses trap heat along the reactor wall by their retardation of the normal convective forces that dissipate heat in the reactor.
It would be highly desirable to have an improved polymerization catalyst system that in a polymerization process would significantly enhance reactor operability and provide an improved polymer product.
2. Description of Related Art
The prior art contains a number of different teachings regarding minimization of fouling and sheeting in commercial scale, continuous olefin polymerization processes. The problem was recognized, along with attempts to resolve it before the widespread, commercial-scale use of metallocene-based catalysts in the late 1980s to early 1990s.
U.S. Pat. No. 4,532,211 used chromium-based metallocenes to treat the reactor bed medium prior to polymerization to prevent charge build-up. In this way, the '211 patent taught the prevention of polymer aggregate formation on the reactor wall and prevention of the subsequent development of “hot spots” at this site and ultimately, sheet formation. Notably, the '211 patent used traditional Zieglar-Natta catalysts such a TiCl4 with MgC12 as the active catalyst. The chromium-based metallocene was present only for its ability to maintain static voltage below those levels that would otherwise result in sheet formation.
For use in similar titanium or vanadium-based catalytic systems, U.S. Pat. No. 4,803,251, to Union Carbide Corporation, disclosed the use of a different group of chemical additives to control the level of static charge accumulation in the reactor. In the '251 case, additives such as alcohols containing up to seven carbon atoms, oxygen and nitric oxide were used in the case of negative electrostatic charge build-up. For positive electrostatic charge build-up, ketones of up to seven carbon atoms were taught. The additives were used at low levels to prevent poisoning of the catalyst; typically ranging from about 0.1 to 25 ppm based on monomer feed. U.S. Pat. No. 5,391,657 also to Union Carbide, expanded on the disclosure of U.S. '251 and used metal containing species as MgO, ZnO, A12O3, CuO and mixtures thereof for the control of positive charge accumulation, and species such as V2O5, SiO2, TiO2, Fe2O3 and mixtures thereof for the control of negative charge accumulation.
Later disclosures were directed toward addressing the problem in metallocene-based catalytic systems. Exxon U.S. Pat. No. 5,436,304 taught a remedy to the sheeting problem that was directed toward alleviating the problem of insufficient heat transfer caused by inadequate fluidization in the fluidized bed. It taught monitoring a condition of the reactor indicative of an onset of a failure condition and controlling the composition of a fluidizing medium or recycle stream in what is commonly known as condensed phase fluidized bed polymerization to correct problems in heat transfer. One way to achieve this was to modify the dew point of the recycle stream. '304 teaches the increase of operating pressure of the reaction/recycle stream and/or the increase in the percentage of condensable fluids and a decrease in the percentage of non-condensable gases in the recycle stream. Thus, the goal was not the prevention of the development of hot spots, but rather their removal or diminution upon evidence of their formation.
U.S. Pat. No. 5,405,922, assigned to Exxon Chemical Patents, teaches the application of remedial measures for sheeting to metallocene-based catalytic systems. Like the '304 patent, it is based upon modifying the dew point of the recycle stream In '922, this is accomplished by the use of non-polymerizable unsaturated hydrocarbons in the recycle stream.
Sheeting and fouling in metallocene-based systems was shown to be reducible in solid supported catalytic systems by the pretreatment via hydrolysis of alumoxane activators prior to their use. It appears that this serves to remove any unreacted alkyl aluminum species. U.S. Pat. No. 5,959,950 teaches that the resulting weight percent of “fines” produced in such cases is decreased, allowing for the realization of decreased fouling and sheeting. U.S. Pat. No. 5,629,253, also to Exxon, discloses a reduction in fouling and sheeting by adding an organometallic compound which is capable of forming alumoxane activators to a water-containing support material. By precisely controlling conditions, most notably temperature, a method is disclosed wherein the mole ratio of the metal of the organometallic compound to the water content of the support material is optimized in such a way as to minimize fouling and sheeting. Precise control of loading was also the essential teaching in U.S. Pat. No. 6,087,291. In contrast to the '253 patent in which the mole ratio of the metal of the organometallic compound to the water content was controlled, the '291 patent teaches a reduction in fouling and sheeting through the optimization of the molar ratio of the metal content of the alumoxane to the transition metal of the metallocene.
More recently, techniques for the reduction in fouling and sheeting in these polymerization processes was extended to catalytic systems utilizing noncoordinating anions (NCA) as activators of metallocenes. U.S. Pat. No. 6,100,214 teaches the use of a amino-modified polymer solid phase that is used to form an association with an NCA and an organometallic transition metal compound, typically metallocene. U.S. Pat. No. 5,863,853 is quite similar, although it teaches the use of porous solid supports into which the catalyst system components are loaded. Like the '291 patent discussed above, precise control of loading was taught, but the optimized parameter was the volume of metallocene and activator solution relative to the pore volume of the porous support.
Sheeting can be a substantial problem in commercial gas phase fluidized bed operation. Although a lot of progress has been made in the manufacture and use of supported metallocene catalysts, there remain important improvements to catalyst operability that might enable:                1. higher reactor utilization.        2. Access to grades that cannot presently be made; and        3. More reliable reactor transitions, e.g. from metallocene to Ziegler-Natta and back.        